1. Field of the Invention
The present invention relates to a stepping motor and more particularly to an arrangement for mounting a coil wound on a bobbin to each of four poles formed on a stator core of a rectangular hybrid stepping motor.
2. Description of the Prior Art
Many conventional permanent magnet hybrid miniangle stepping motors are of such a type as having a step angle of 1.8.degree. or 3.6.degree.. The arrangement of the stepping motors of the type mentioned above is such that, as shown in FIG. 1, eight poles 2 are formed projecting at symmetrical positions along the inner peripheral edge of a core 1, each pole 2 being wound with a coil. In this arrangement, if the outer shape of the core 1 is rectangular, there is a portion 3 at each of the corners which does not function as a core. Moreover, there is provided a smaller slit area for winding a coil between adjacent poles 2, since the eight poles 2 are projected from the inner peripheral edge of the core. Furthermore, since the poles 2 are directed toward the center of the core 1, there is provided a narrower gap 5 between the ends of the adjacent poles 2 as an opening of each of slit portions 4 for winding a coil. Accordingly, it is difficult to wind a coil on each pole 2, so that the workability is not good, resulting in not only a higher cost but also incapability of increase in production.
Therefore, a stepping motor employing a rectangular core with a substantially square shape has been proposed, such as stated in a U.S. Pat. No. 4,234,808. This stepping motor is arranged such that, as shown in FIG. 2, each of poles 9 is projected from the inner side edge 8 of each of sides 7 of a substantially square core 6 in order to enlarge the gap between each pair of adjacent poles 9 thereby facilitating winding of a coil. In the stepping motor of the type mentioned above, however, no consideration is taken at all for the relationship between the height of each pole of the core and the outside diameter of a rotor. Therefore, the stepping motor has a shortcoming that the coil winding amount with respect to the core is smaller and consequently, the electric loading is smaller, resulting in a smaller torque. Moreover, the conventional stepping motor shown in FIG. 2 has another shortcoming of bad workability in mounting a coil, since the stepping motor employs a method wherein a coil conductor coated with an insulation film is annularly wound on each pole 9 of the core 6 or an arrangement such that an insulation cylinder split into two is fitted on each pole 9 and wound with a coil conductor.
Further, in the conventional stepping motor shown in FIG. 2, a plurality of projections formed at the ends of some poles in the poles 9 each projected from the inner side edge 8 of each side 7 of the core 6 are offset sideways from the widthwise center of the corresponding side 7 according to a step angle in connection with the projections formed on the periphery of the rotor and a step angle. Therefore, each pole 9 having the projection offset sideways from the widthwise center of the corresponding side 7 is not formed so as to be symmetrical with respect to the widthwise center of the inner side edge 8 of the corresponding side 7, so that the pole 9 is offset sideways from the widthwise center of the side 7. Accordingly, when a coil 10 is wound on the pole 9, a useless space 11 is formed on the side opposite to the side to which the pole 9 is offset. Such tendency is particularly remarkable in the case where the number of the projections of the rotor corresponding to a step angle is odd. In such a case, the coil winding amount with respect to the core is reduced. Accordingly, the conventional stepping motor shown in FIG. 2 has a shortcoming of a smaller electric loading as well as a smaller torque correspondingly.
Furthermore, the stepping motor shown in FIG. 2 has still another shortcoming of a poor assembly workability because of such an arrangement that each coil 10 is wound on the corresponding pole 9 of the core 6 through an insulation sheet, so that it is troublesome to mount the coils 10.
Furthermore, the conventional stepping motor having each pole projected from the inner side edge of a rectangular stator core, as shown in FIG. 2, has such a shortcoming that a magnetic saturation takes place at the pole portions of the core, and particularly when an axial magnetic leakage occurs, a magnetically adverse effect is exerted on the apparatus incorporating the motor.